You can actually buy that costume on eBay. Quite a few stores on there sell it.
Otherwise, just find a Miku tutorial and follow that - it's literally the same outfit.
SakuraCon 2015/Otakon 2015:
Serra - Fire Emblem: Rekka no Ken (may be delayed due to school)
Ishtar - Fire Emblem: Genealogy of the Holy War
Princess Zelda - Hyrule Warriors
Say'ri - Fire Emblem: Awakening
Nepeta Leijon - Homestuck (delayed due to school)